case-sensitivity of filesystem apis

[programmerjake]

I think it would be a good idea to standardise on if the file-system APIs should be case-sensitive or not and to what extent implementations should enforce case-sensitive/insensitive file-system access on common platforms such as Linux or Windows.

See https://github.com/CraneStation/wasmtime/pull/235#issuecomment-518897502 for some ideas.